serial: uart_native_tty: Polling thread fix

Addresses an issue where attempts to transmit data fail.
The identified cause of this failure is the handling of the
`rx_irq_enable` and `tx_irq_enable` flags. The `else if` handling the
`tx_irq_enable` was replaced by an `if` so both flags can be true,
and both functions can be called. This fix was tested with the
`cellular_modem` example successfully.

Signed-off-by: Fabian Kainka <kainka@cognid.de>